Optical device

ABSTRACT

An optical device wherein an optical waveguide is formed on a dielectric substrate, the optical device includes an input part and an output part where the optical waveguide and corresponding optical fibers are connected. A stress layer is provided for at least one of the input part and the output part. The stress layer applies a stress to the optical waveguide so that an index of refraction of the optical waveguide is reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to optical devices, and morespecifically, to an optical device used in technical fields such asoptical communication or optical signal processing.

2. Description of the Related Art

As for recent and continuing development of optical communicationsystems, a system having a large capacity and high functions has beenrequested. Hence, it is required to develop an optical device such as anoptical switch or modulator for high speed optical wave. In such anoptical device, not only high speed but also low loss or low voltage isrequired.

As the high speed optical device, for example, an optical switch, anoptical modulator, and an optical control element are reported wherein awaveguide is formed in a substrate made of a crystal of lithium niobate(LiNbO₃) having a large electrooptic coefficient and the change of theindex of refraction of the waveguide is controlled by changing it in anelectric field by using the electrooptic effect.

The optical waveguide having a structure where a metal such as titanium(Ti) or the like is diffused in the crystal of lithium niobate (LiNbO₃)obtains low propagation loss equal to or less than 0.1 dB/cm for awavelength of 1550 nm.

However, in order to use the optical device using such a diffusionwaveguide (optical waveguide) in an optical fiber transmission system,it is necessary to consider the coupling loss with the optical fiber.

In addition, in a case where the optical waveguide isthree-dimensionally formed on the dielectric substrate by using afemto-second laser or high intensity laser, it is well known that thecross section of the waveguide becomes elliptic. Because of this, it isnecessary to consider the coupling loss similar to the above-mentioneddiffusion waveguide.

FIG. 1 is a schematic view showing a light intensity distribution oflight propagating in an optical fiber and a light intensity distributionof light propagating in a diffusion waveguide of an optical waveguideelement, in the related art.

The intensity (mode) distribution of the light propagating in theoptical fiber is circular shaped as shown in FIG. 1-(a). On the otherhand, in the optical device using the diffusion waveguide, as shown inFIG. 1-(b), the distribution of the indices of refraction between thedirection perpendicular to the substrate, namely substrate depthdirection, and the direction parallel to the substrate are different.The intensity distribution of the light is extremely different from thecircular shape and is substantially elliptical in shape.

FIG. 2 provides graphs of the distribution of the change of the index ofrefraction in the direction perpendicular to the substrate, namely thesubstrate depth direction, and parallel to the horizontal with thesubstrate, in the related art optical device.

The intensity distribution of the light in the diffusion waveguide isdefined by a principle of the diffusion. The diffusion waveguidereceives diffusing atoms from an upper side. Hence, the distribution ofthe index of refraction horizontal (parallel) to the substrate has, asshown in FIG. 2-(b), a substantially symmetric shape. On the other hand,the distribution of the change of the index of refraction in thedirection perpendicular to the substrate, namely the substrate depthdirection is, as shown in FIG. 2-(a), leans (is skewed) toward thesurface of the substrate. Because of this, when the diffusion waveguideis inserted between the optical fibers, coupling loss of approximately 2dB in total at the input and output parts may be incurred.

Therefore, in order to reduce the coupling loss between the diffusionwaveguide and the optical fibers, it is necessary to make thedistribution of the change of the index of refraction of the diffusionwaveguide at the optical input and output parts similar to thesubstantially circular-shaped light intensity distribution of theoptical fiber.

On the other hand, in the optical control part, it is possible to obtainhigh electric field application efficiency when the light intensitydistribution leans toward the vicinity of an electrode having a highstrength applied electronic field.

In order to solve the above-discussed problems a method is suggested inJapanese Laid Open Patent Application Publication No. 62-103604 wherebya titanium (Ti) diffusion waveguide is formed and then magnesium oxide(MgO) is additionally diffused so that the index of refraction of thesurface is decreased.

In addition, Japanese Laid Open Patent Application Publication No.2005-284256 discloses a waveguide-type optical splitter having astructure where a waveguide for input, plural waveguides for output anda slab waveguide are formed on a substrate. The slab waveguide has anincident end and an output end. The output end has a circular shapecentered around the incident end or its vicinity. The waveguide forinput is connected to the incident end and the plural waveguides foroutput are connected to the output end. The waveguide for input isconnected to the incident end via a waveguide, whose opening width isnarrowed and tapered.

Furthermore, Japanese Patent No. 2793562 discloses a structure where anoptical waveguide is formed on a substrate of lithium niobate (LiNbO₃)and the optical waveguide is formed by effecting thermal diffusion oftitanium (Ti).

In addition, Japanese Patent No. 2817769 discloses a semiconductoroptical amplifier device including a semiconductor laser section and asemiconductor optical amplifier both of which are formed on the samesemiconductor substrate and which are coupled to each other, thesemiconductor optical amplifier including a waveguide layer formed onthe semiconductor substrate and a tapered electrode formed on an uppersurface thereof, the semiconductor optical amplifier being supplied withan incident laser beam from the semiconductor laser section through anincident surface and amplifying the incident laser beam to emit anamplified laser beam as an output laser beam through an emissionsurface, the tapered electrode spreading toward the emission surface.

However, in the process discussed in Japanese Laid Open PatentApplication Publication No. 62-103604, after titanium (Ti) is diffused,deposition and thermal diffusion of magnesium oxide (MgO) are requiredand therefore the number of the processes is increased.

Furthermore, in a case where lithium niobate (LiNbO₃) is used as adielectric substrate, out diffusion of lithia (Li₂O) from inside oflithium niobate (LiNbO₃) is expected due to the diffusion of magnesiumoxide (MgO). Therefore, it is difficult to achieve a desirablerefractive index profile.

In addition, for example, if the out diffusion of lithia (Li₂O) happenstoo much, the ratio of lithium (Li) and niobium (Nb) in a crystal may bechanged and therefore a decrease of electrooptic coefficient would beexpected.

SUMMARY OF THE INVENTION

Accordingly, the present invention may provide a novel and usefuloptical device solving one or more of the problems discussed above.

Another and more specific object of the present invention may be toprovide an optical device having a structure where the light intensitydistribution of an optical waveguide is substantially consistent withthe light intensity distribution of an optical fiber for at least one ofoptical input and output parts (parts connecting the optical device andthe optical fiber) and therefore there is little coupling loss.

The above object of the present invention is achieved by an opticaldevice wherein an optical waveguide is formed on a dielectric substrate,the optical device including an input part and an output part where theoptical waveguide and corresponding optical fibers are connected;wherein a stress layer is provided for at least one of the input partand the output part; and the stress layer applies a stress to theoptical waveguide so that an index of refraction of the opticalwaveguide is reduced.

The stress layer may be formed in a taper shape; and the stress appliedto the optical waveguide may be weakened as a position approaches fromthe at least one of the input part and the output part to the controlpart where an electrode is provided. Width of the stress layer may beformed in a taper shape. Thickness of the stress layer may be formed ina taper shape.

The optical waveguide may be put between a plurality of the stresslayers at the at least one of the input part and the output part, in acase where the dielectric substrate is made of a material having apositive photoelastic coefficient and a coefficient of thermal expansionof the stress layer is larger than a coefficient of thermal expansion ofthe dielectric substrate.

The optical waveguide may be put between a plurality of the stresslayers at the at least one of the input part and the output part, in acase where the dielectric substrate is made of a material having anegative photoelastic coefficient and a coefficient of thermal expansionof the stress layer is smaller than a coefficient of thermal expansionof the dielectric substrate.

The stress layer may be provided on the optical waveguide at the atleast one of the input part and the output part, in a case where thedielectric substrate is made of a material having a positivephotoelastic coefficient and a coefficient of thermal expansion of thestress layer is smaller than a coefficient of thermal expansion of thedielectric substrate.

The stress layer may be provided on the optical waveguide at the atleast one of the input part and the output part, in a case where thedielectric substrate is made of a material having a negativephotoelastic coefficient and a coefficient of thermal expansion of thestress layer is larger than a coefficient of thermal expansion of thedielectric substrate.

According to an embodiment of the present invention, it is possible toprovide the optical device having the structure where the lightintensity distribution of the optical waveguide is substantiallyconsistent with the light intensity distribution of the optical fiberfor at least one of the optical input and output parts (the partsconnecting the optical device and the optical fibers) and thereforethere is little coupling loss.

Other objects, features, and advantages of the present invention will become more apparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a light intensity distribution oflight propagating in an optical fiber and a light intensity distributionof light propagating in a diffusion waveguide of an optical waveguideelement, in the related art;

FIG. 2 is a graph of distribution of change of an index of refraction ina direction perpendicular to or parallel to a substrate in the relatedart optical device;

FIG. 3 is a plan view showing a schematic structure of an optical deviceof a first embodiment of the present invention;

FIG. 4 is a schematic view seen in a direction shown by an arrow A ofthe optical device shown in FIG. 3;

FIG. 5 is a perspective view of an optical device of a modified exampleof the first embodiment of the present invention;

FIG. 6 is a plan view showing a schematic structure of an optical deviceof a second embodiment of the present invention;

FIG. 7 is a schematic view seen in a direction shown by an arrow A ofthe optical device shown in FIG. 6;

FIG. 8 is a perspective view of an optical device of a modified exampleof the second embodiment of the present invention;

FIG. 9 is a table with respect to combinations of a material used forthe substrate and a material used for a stress layer and applications ofstructures of the embodiments of the present application to thecombinations;

FIG. 10 is a graph showing a distribution of change of the index ofrefraction in the direction perpendicular to the substrate in theembodiment of the present invention;

FIG. 11 is a graph showing a light intensity distribution of a diffusionwaveguide of the embodiment of the present invention by comparing therelated art;

FIG. 12 is a cross-sectional view of input and output parts of anoptical device of a third embodiment of the present invention; and

FIG. 13 is a plan view showing a schematic structure of the opticaldevice of the third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description is given below, with reference to the FIG. 3 through FIG.13 of embodiments of the present invention.

1. First Embodiment of the Present Invention

FIG. 3 is a plan view showing a schematic structure of an optical deviceof a first embodiment of the present invention. FIG. 4 is a schematicview seen in a direction shown by an arrow A of the optical device shownin FIG. 3.

In FIG. 3 and FIG. 4, an example where the present invention is appliedto a Mach-Zehnder type optical modulator is shown as an optical device10.

Referring to FIG. 3, the Mach-Zehnder type optical modulator is formedas the optical device 10. The optical device 10 has a structure where awaveguide (optical waveguide) 3 is formed on a dielectric substrate 1having a photoelastic effect such as lithium niobate (LiNbO₃). Thewaveguide (optical waveguide) 3 is made by diffusing metal atoms such astitanium (Ti).

The dielectric substrate 1 made of lithium niobate (LiNbO₃), as well asa substrate made of lithium tantalate (LiTaO₂) discussed below, havelarge electrooptic coefficients. Hence, it is possible to form adiffusion waveguide having low loss on the dielectric substrate 1.Furthermore, as a material for being diffused on the dielectricsubstrate 1, in addition to titanium (Ti), zinc (Zn), nickel (Ni), orthe like may be used.

The optical device 10 includes an input part, a control part, and anoutput part.

In the control part, modulating electrodes 4-1 and 4-2 are provided inparallel. By applying a radio frequency (RF) voltage to the modulatingelectrodes 4-1 and 4-2, the optical path length of light is changed dueto an electrooptic effect so that intensity modulation of the light isperformed.

In the input part and the output part, the stress layers 2-1 and 2-2 arepatterned and provided so that the diffusion waveguide 3 is put betweenthe stress layers 2-1 and 2-2.

Referring to FIG. 4, the stress layers 2-1 and 2-2 make the diffusionwaveguide 3 put between the stress layers 2-1 and 2-2 have a stressinducing the change of the index of refraction due to a photoelasticeffect.

In other words, a stress “σ×t (t: thickness of the stress layers 2-1 and2-2)” is given to the diffusion waveguide 3 put between the stresslayers 2-1 and 2-2 so that expansion strain acts on a surface of thediffusion waveguide 3. As a result of this, the change of the index ofrefraction is induced due to a photoelastic effect and therefore theindex of refraction of the surface of the substrate 1 is decreased.

Under this structure, the light intensity distribution of the diffusionwaveguide 3 in the input part and the output part can be made to havesimilar substantially circular shapes, the circular shape being thelight intensity distribution of the optical fibers. As a result of this,it is possible to reduce the coupling loss of the optical fibers and theinput and output parts.

Referring back to FIG. 3, it is necessary to improve the applicationefficiency of the electric field in the control part. Therefore, it ispreferable to make the light intensity distribution lean toward thevicinities of the electrodes 4-1 and 4-2 having large strengths of theapplication electric fields, namely make the light intensitydistribution lean toward the surface of the substrate like a normaldiffusion waveguide.

Because of this, the stress layers 2-1 and 2-2 are provided only at theinput and output parts so that a decreasing degree of the index ofrefraction is most at the input and output parts. On the other hand, thestress layers 2-1 and 2-2 are not provided at the control part.

At the input and output parts, the width of the stress layers 2-1 and2-2 are formed in taper shapes so as to be narrower as a positionapproaches the control part. As a result of this, the stresses appliedby the stress layers 2-1 and 2-2 are gradually weakened and changed sothat the coupling loss is prevented.

In the meantime, while the width of the stress layers 2-1 and 2-2 areformed in taper shapes so as to be narrower as the position approachesthe control part in the example shown in FIG. 3 as discussed above, thepresent invention is not limited to this. The present invention may beapplied to an example shown in FIG. 5. Here, FIG. 5 is a perspectiveview of an optical device of a modified example of the first embodimentof the present invention.

Referring to FIG. 5, in this example, thicknesses of stress layers 2′-1and 2′-2 are formed in taper shapes at the input and output parts so asto be thinner as the position approaches the control part. In thestructure of this example, the stresses applied by the stress layers2′-1 and 2′-2 can be gradually weakened and changed so that the sameeffect as that of the example shown in FIG. 3 can be achieved.

Meanwhile, the stress layers 2-1 and 2-2 can be formed by, for example,an evaporation method or a sputtering method. It is preferable to formthe stress layers 2-1 and 2-2 as thin films at approximately 50 through500° C.

Here, “α_(s)” represents a coefficient of thermal expansion of thesubstrate 1. “α_(f)” represents coefficients of thermal expansion of thestress layers 2-1 (2′-1) and 2-2 (2′-2). The stress layers 2-1 (2′-1)and 2-2 (2′-2) having thickness of “t” μm are formed on the substrate 1at “T” ° C. When the temperature is turned back to room temperature(normal temperature) T₀, a stress “σ_(th)” is generated at the substrate1 due to the difference of the coefficients of thermal expansion. The“σ_(th)” is calculated by the following formula 1.

$\begin{matrix}{\sigma_{th} = \frac{E_{f}\Delta\;{T( {\alpha_{s} - \alpha_{f}} )}}{1 - v_{f}}} & \lbrack {{Formula}\mspace{14mu} 1} \rbrack\end{matrix}$

Here, ΔT=(T−T₀). In addition, E_(f) represents Young's modulus and ν_(f)represents Poisson's ratio of the stress layer.

In addition, the stress σ when the thin film is formed is generallyexpressed as “σ=σ_(int)+σ_(th)”. Here, σ_(int) is called a true(intrinsic) stress and is a stress at the time of deposition not basedon heat. As discussed above, “σ_(th)” is a stress generated by thedifference of the coefficients of thermal expansion.

As discussed above, the strain S is generated at the diffusion waveguide3 put between the stress layers 2-1 (2′-1) and 2-2 (2′-2). Therelationship between the strain S and the stress σ is found bycalculating an elastic equation with an elastic constant of thesubstrate 1.

When the value of the stress σ is positive, the generated strain S ispositive. When the value of the stress σ is negative, the generatedstrain S is negative. The value of the stress σ is proportional to thevalue of the strain S.

In addition, if the strain S is applied to the substrate 1 having anelectrooptic effect, the index of refraction is changed. The change ofthe index of refraction due to the electrooptic effect is calculated bythe following formula 2.

$\begin{matrix}{{\Delta\; n} = {{- \frac{1}{2}}n^{3}{pS}}} & \lbrack {{Formula}\mspace{14mu} 2} \rbrack\end{matrix}$

Here, “n” represents the index of refraction. “S” represents the strainapplied to the substrate. “p” represents a photoelastic coefficient.

For example, when the substrate 1 is made of a material having apositive photoelastic coefficient such as lithium niobate (LiNbO₃), amaterial (for example, polyimide, aluminum (Al), or the like) having acoefficient of thermal expansion larger than that of lithium niobate(LiNbO₃) (α_(s)<α_(f)) is selected for the stress layers 2-1 (2′-0) and2-2 (2′-2). By generating the expansion stress σ having a positive valueat the substrate 1 and making the generated strain S have a positivevalue (expansion strain), it is possible to decrease the index ofrefraction (namely to make Δn have a negative value).

On the other hand, when the substrate 1 is made of a material having anegative photoelastic coefficient such as gallium arsenide (GaAs), amaterial (for example, silicon oxide (SiO₂), silicon nitride (SiN), orthe like) having a coefficient of thermal expansion smaller than that ofgallium arsenide (GaAs) (α_(s)>α_(f)) is selected for the stress layers2-1 (2′-1) and 2-2 (2′-2). As a result of this, it is possible todecrease the index of refraction (namely to make Δn have a negativevalue).

Thus, it is possible to generate the stress decreasing the index ofrefraction by a thermal stress or the like due to the deposition of thethin film and possible to reduce the process of additional diffusion. Inaddition, since there is no damage received by the substrate, theelectrooptic coefficient or out diffusion is not changed. Hence, it ispossible to easily obtain a desirable distribution of the index ofrefraction.

2. Second Embodiment of the Present Invention

FIG. 6 is a plan view showing a schematic structure of an optical deviceof a second embodiment of the present invention. FIG. 7 is a schematicview seen in a direction shown by an arrow A of the optical device shownin FIG. 6. In FIG. 6 and FIG. 7, parts that are the same as the partsshown in the first embodiment of the present invention are given thesame reference numerals, and explanation thereof is omitted.

In the above-discussed first embodiment of the present invention, asshown in FIG. 4, the stress layers 2-1 and 2-2 are patterned andprovided at the input and output parts so that the diffusion waveguide 3is put between the stress layers 2-1 and 2-2. On the other hand, in thesecond embodiment of the present invention, as shown in FIG. 6 and FIG.7, patterning reverse of the patterning in the first embodiment of thepresent invention is made. A stress layer 22 is provided on thediffusion waveguide 3 and makes the diffusion waveguide 3 have a stressinducing the change of the index of refraction due to the photoelasticeffect.

In other wards, a stress “σ×t (t: thickness of the stress layer 22)” isapplied to the diffusion waveguide 3 so that contraction strain acts ona surface of the diffusion waveguide 3. As a result of this, the changeof the index of refraction is induced due to the electrooptic effect andtherefore the index of refraction of the surface of the substrate 1 isdecreased.

In the second embodiment of the present invention, as well as the firstembodiment of the present invention, it is necessary to improve theapplication efficiency of an electric field in the control part.Therefore, it is preferable to make the light intensity distributionlean toward the vicinities of the electrodes 4-1 and 4-2 having largestrengths of the application electric fields, namely make the lightintensity distribution lean toward the surface of the substrate like anormal diffusion waveguide.

Because of this, the stress layer 22 is provided only at the input andoutput parts and not provided at the control part.

At the input and output parts, the width of the stress layers 22 areformed in taper shapes so as to be narrower as a position approaches tothe control part. As a result of this, the stress applied by the stresslayer 22 is gradually weakened and changed so that the coupling loss isprevented.

FIG. 8 is a perspective view of an optical device of a modified exampleof the second embodiment of the present invention.

Referring to FIG. 8, in the second embodiment of the present invention,as well as the first embodiment of the present invention, as a modifiedexample of the example shown in FIG. 6, the thickness of stress layer 27may be formed in a taper shape at the input and output parts so as to bethinner as the position approaches the control part. In a structure ofthis example, the stresses applied by the stress layer 27 can begradually weakened and changed so that the same effect as that of theexample shown in FIG. 6 can be achieved.

In this example, unlike the first embodiment of the present invention,when the substrate 1 is made of a material having a positivephotoelastic coefficient, a material having a coefficient of thermalexpansion smaller than that of the substrate 1 is selected for thestress layers 22 (27) (α_(s)>α_(f)). By generating the constrictionstress σ having a negative value at the substrate 1 and making thegenerated strain S have a negative value (constriction strain), it ispossible to decrease the index of refraction.

On the other hand, when the substrate 1 is made of a material having anegative photoelastic coefficient, a material having a coefficient ofthermal expansion smaller than that of the substrate 1 (α_(s)<α_(f)) isselected as the stress layers 22 (27). As a result of this, it ispossible to decrease the index of refraction.

Here, a table with respect to combinations of a material used for thesubstrate and a material used for a stress layer and applications ofstructures of the embodiments of the present application to thecombinations is shown in FIG. 9.

FIG. 9-(a) is a table showing photoelastic coefficients “p” andcoefficients of thermal expansion “α_(s) (1/° C.)” of materials used forthe substrate 1.

For example, in a case where the substrate 1 is made of lithium niobate(LiNbO₃), the photoelastic coefficient “p” is positive and thecoefficient of thermal expansion “π_(s)” is 15×10⁻⁶ (1/° C.). In a casewhere the substrate 1 is made of lithium tantalate (LiTaO₃), thephotoelastic coefficient “p” is positive. The cases where the substrate1 is made of lithium niobate (LiNbO₃) and the case where the substrate 1is made of lithium tantalate (LiTaO₃) are classified as a classification“A”. In a case where the substrate 1 is made of gallium arsenide (GaAs),the photoelastic coefficient “p” is negative. The case where thesubstrate 1 is made of gallium arsenide (GaAs) is classified as aclassification “B”.

FIG. 9-(b) shows coefficients of thermal expansion “α_(f)” of materialsapplied as the stress layer 2 (2′) or 22 (27) and combinations of theclassifications of the applied substrate and corresponding embodimentsof the present application.

In FIG. 9-(b), as examples of the materials for the stress layer 2 (2′)or 22 (27), aluminum (Al), polyimide, silicon oxide (SiO₂), and nitridesilicon (SiN) are used.

For example, in a case where (1) aluminum (Al) having coefficient ofthermal expansion “α_(f)” of 27×10⁻⁶ (1/° C.) is applied as the materialof the stress layer; and (2) the substrate 1 belongs to theclassification A, that is the substrate 1 is made of lithium niobate(LiNbO₃) or lithium tantalate (LiTaO₃) having the positive photoelasticcoefficients “p”, the structure of the first embodiment of the presentinvention discussed with reference to FIG. 3 through FIG. 5 is applied.

In addition, in a case where (1) aluminum (Al) is applied as thematerial of the stress layer; and (2) the substrate 1 belongs to theclassification B, that is the substrate 1 is made of gallium arsenide(GaAs) having the negative photoelastic coefficients “p”, the structureof the second embodiment of the present invention discussed withreference to FIG. 6 through FIG. 8 is applied.

In either case, the change of the index of refraction can be induced bythe electrooptic effect and the reducing degree of the index ofrefraction can be made large, on the surfaces of the input part and theoutput part. In addition, in either case, the light intensitydistribution can be leaned toward the vicinities of the electrodes 4-1and 4-2 by the control part.

Next, an effect of the embodiments of the present invention is discussedwith reference to FIG. 10 and FIG. 11. Here, FIG. 10 is a graph showinga distribution of change of the index of refraction in the directionperpendicular to the substrate in the embodiment of the presentinvention. FIG. 11 is a graph showing a light intensity distribution ofa diffusion waveguide of the embodiment of the present invention bycomparing the related art.

First, referring to FIG. 10, a dashed line shows a distribution shown inFIG. 2-(a), namely a distribution of the change of the index ofrefraction in a direction perpendicular to the substrate (substratedepth direction) of the optical device having no stress layer 2 (2′) or22 (27) of the embodiments of the present invention.

As discussed above, in a case where the stress layer 2 (2′) or 22 (27)is not provided, distribution of the change of the index of refractionin the direction perpendicular to the substrate (substrate depthdirection) leans toward the surface of the substrate.

However, by providing the stress layer 2 (2′) or 22 (27) of theembodiments of the present invention, as shown by the one-dotted line,the change of the index of refraction due to the stress layer 2 (2′) or22 (27) is generated. As a result of this, as shown by a solid line,change of the index of refraction being substantially symmetrical withrespect to a peak point can be obtained.

The distribution of the light intensity in the direction perpendicularto the substrate (substrate depth direction), the distributioncorresponding to the change of the index of refraction, is shown in FIG.11.

More specifically, the distribution of the light intensity of theoptical fiber and the distribution of the light intensity at thediffusion waveguide in a case where the stress layer 2 (2′) or 22 (27)is not provided, are shown in FIG. 11-(a). The distribution of the lightintensity of the optical fiber and the distribution of the lightintensity at the diffusion waveguide in a case where the stress layer 2(2′) or 22 (27) is provided, are shown in FIG. 11-(b).

In FIG. 11, a dashed line shows the distribution of the light intensityof the optical fiber and a solid line shows the distribution of thelight intensity at the diffusion waveguide.

Referring to FIG. 11-(a), in the case where the stress layer 2 (2′) or22 (27) is not provided, the distribution of the light intensity at thediffusion waveguide leans toward the surface of the substrate. Hence,the distribution of the light intensity at the diffusion waveguide isnot consistent with the distribution of the light intensity of theoptical fiber.

Referring to FIG. 11-(b), in the case where the stress layer 2 (2′) or22 (27) is provided, as the distribution of the light intensity at thediffusion waveguide, a distribution corresponding to the change of theindex of refraction shown in FIG. 10, is obtained. Hence, thedistribution of the light intensity at the diffusion waveguide issubstantially consistent with the distribution of the light intensity ofthe optical fiber.

The coupling loss can be expressed by an overlap integral of thedistribution of the light intensity of the optical fiber and thedistribution of light intensity of the diffusion waveguide.

In the case where the stress layer 2 (2′) or 22 (27) is provided, thedistribution of the light intensity of the diffusion waveguide becomessimilar to the substantially circular shaped distribution beingsubstantially symmetric with respect to a peak point.

Therefore, the overlap of the distribution of the light intensity of thediffusion waveguide and the distribution of the light intensity of theoptical fiber becomes large so that the coupling loss is decreased.

Thus, in the above-discussed embodiments of the present invention, sincethe stress layer 2 (2′) or 22 (27) is provided at the input and outputparts, namely a part connecting the optical device and the opticalfiber, the distribution of the light intensity at the diffusionwaveguide is made substantially consistent with the correspondingdistribution of light intensity of the optical fiber so that thecoupling loss with the optical fiber can be made small.

In addition, since the stress layer 2 (2′) or 22 (27) is provided at thecontrol part, the light intensity can be leaned toward the vicinity ofthe electrode on the surface so that high electric field applicationefficiency can be obtained.

3. Third Embodiment of the Present Invention

In the above-discussed embodiments of the present invention, the presentinvention is applied to the waveguide (mainly, diffusion waveguide)wherein the distribution of the light intensity is leaned toward thesurface of the substrate.

However, the present invention is not limited to these examples. Thepresent invention may be applied to other asymmetric optical waveguides.For example, a three dimensional optical waveguide can be formed at anoptical depth of the substrate by making convergent radiotherapy of ahigh intensity laser such as Femto second laser to the dielectricsubstrate. In this case, generally, the formed optical waveguide has anelliptic-shaped configuration.

An example of a case where the present invention is applied to anelliptic waveguide having optional depth is shown in FIG. 12. Here, FIG.12 is a cross-sectional view of input and output parts of an opticaldevice of a third embodiment of the present invention.

At the input and output parts, groove forming parts 33-1 and 33-2 areformed in the dielectric substrate 1 by dry etching such as RIE(Reactive Ion Etching). Stress layers 32-1 and 32-2 are stacked in thegroove forming parts 33-1 and 33-2.

As shown in FIG. 12, by stresses applied to side surface parts of thegroove forming parts 33-1 and 33-2, it is possible to adjust thedistribution of the light intensity to a substantially circular shapeand reduce the coupling loss with the optical fiber. Here, as shown inFIG. 12, layer thickness “t” of the stress layers 32-1 and 32-2 aresubstantially constant (the stress is proportional to the layerthickness.).

In this case, as shown in FIG. 13, the width of the stress layers 32-1and 32-2 can be formed in the taper shapes. Here, FIG. 13 is a plan viewshowing a schematic structure of the optical device of the thirdembodiment of the present invention.

A selection way and structure of the stress layers 32-1 and 32-2 can beimplemented in the same way as the first and second embodiments of thepresent invention.

It should be noted that the examples shown in FIG. 12 and FIG. 13 arejust examples of the present invention. By combining embodiment of thepresent invention, it is possible to make any elliptic waveguide have asubstantially circular shaped light intensity distribution.

The present invention is not limited to these embodiments, butvariations and modifications may be made without departing from thescope of the present invention.

For example, in the above-discussed embodiments of the presentinvention, the width or thickness of the stress layer 2 (22) or 2′ (27)is formed in the taper shape so as to be narrower or thinner as aposition approaches the control part. However, the present invention isnot limited to these examples. As long as the stress applied by thestress layer 2 (22) or 2′ (27) can be gradually weakened and changed asthe position approaches the control part, there is no limitation of theangle or configuration of the taper.

In addition, a buffer layer formed by a thin film made of, for example,silicon oxide (SiO₂) may be provided between the modulating electrodes4-1 and 4-2 and the upper surface of the substrate 1 where the diffusionwaveguide 3 is provided. By such a buffer layer, it is possible toprevent absorption of light due to the modulating electrodes 4-1 and4-2.

This patent application is based on Japanese Priority Patent ApplicationNo. 2006-123701 filed on Apr. 27, 2006, the entire contents of which arehereby incorporated by reference.

1. An optical device wherein an optical waveguide is formed on adielectric substrate, the optical device comprising: an input part andan output part where the optical waveguide and corresponding opticalfibers are connected; wherein a stress layer is provided for at leastone of the input part and the output part; the stress layer applies astress to the optical waveguide so that an index of refraction of theoptical waveguide is reduced; the stress applied to the opticalwaveguide is weakened as a position approaches from the at least one ofthe input part and the output part to a control part where an electrodeis provided; and wherein thickness of the stress layer is formed in ataper shape.
 2. An optical device wherein an optical waveguide is formedon a dielectric substrate, the optical device comprising: an input partand an output part where the optical waveguide and corresponding opticalfibers are connected; wherein a stress layer is provided for at leastone of the input part and the output part; the stress layer applies astress to the optical waveguide so that an index of refraction of theoptical waveguide is reduced; the thickness of the stress layer isformed in a taper shape; and the stress applied to the optical waveguideis weakened as a position approaches from the at least one of the inputpart and the output part is provided.
 3. The optical device as claimedin claim 2, wherein the stress layer reduces the index of refraction inthe vicinity of a surface of the at least one of the input part and theoutput part.
 4. The optical device as claimed in claim 2, wherein thestress applied to the optical waveguide is formed by a thermal stress ofthe stress layer.
 5. The optical device as claimed in claim 2, whereinthe stress applied to the optical waveguide is weakened as a positionapproaches from the at least one of the input part and the output partto a control part where an electrode is provided.
 6. The optical deviceas claimed in claim 5, wherein width of the stress layer is formed in ataper shape.
 7. The optical device as claimed in claim 2, wherein thedielectric substrate is made of lithium niobate or lithium tantalate;and the stress layer is made of aluminum or polyimide.
 8. The opticaldevice as claimed in claim 2, wherein the optical waveguide is putbetween a plurality of the stress layers at the at least one of theinput part and the output part, in a case where the dielectric substrateis made of a material having a negative photoelastic coefficient and acoefficient of thermal expansion of the stress layer is smaller than acoefficient of thermal expansion of the dielectric substrate.
 9. Theoptical device as claimed in claim 8, wherein the dielectric substrateis made of gallium arsenide; and the stress layer is made of siliconoxide or silicon nitride.
 10. The optical device as claimed in claim 2,wherein the stress layer is provided on the optical waveguide at the atleast one of the input part and the output part, in a case where thedielectric substrate is made of a material having a positivephotoelastic coefficient and a coefficient of thermal expansion of thestress layer is smaller than a coefficient of thermal expansion of thedielectric substrate.
 11. The optical device as claimed in claim 10,wherein the dielectric substrate is made of lithium niobate or lithiumtantalate; and the stress layer is made of silicon oxide or siliconnitride.
 12. The optical device as claimed in claim 2, wherein thestress layer is provided on the optical waveguide at the at least one ofthe input part and the output part, in a case where the dielectricsubstrate is made of a material having a negative photoelasticcoefficient and a coefficient of thermal expansion of the stress layeris larger than a coefficient of thermal expansion of the dielectricsubstrate.
 13. The optical device as claimed in claim 12, wherein thedielectric substrate is made of gallium arsenide; and the stress layeris made of aluminum or polyimide.
 14. The optical device as claimed inclaim 2, wherein the optical waveguide is put between a plurality of thestress layers at the at least one of the input part and the output part,in a case where the dielectric substrate is made of a material having apositive photoelastic coefficient and a coefficient of thermal expansionof the stress layer is larger than a coefficient of thermal expansion ofthe dielectric substrate.